lxc-destroy: Remove container with all snapshots

lxc-destroy: Remove container with all snapshots

- This enables the user to destroy a container with all its snapshots without
  having to use lxc-snapshot first to destroy all snapshots. (The enum values
  DESTROY and SNAP from the previous commit are reused here again.)
- Some unification regarding the usage of exit() and return has been done.

Signed-off-by: Christian Brauner <christianvanbrauner@gmail.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>